If I get up in the middle of the night and go to the toilet (in our en suite), flush the toilet, etc., my husband never wakes up. But if I whisper his name in a tiny voice, 'Mat, Mat,' he wakes up straight away.
That's probably another example of how, without your realising it, your brain processes all the sounds you hear, but you only become aware of sounds if your brain has 'decided' they are important to you.
Here's another example to think about. Imagine someone walks past you, whistling. You don't pay any attention to it and soon forget it.
But if a friend was walking with you and said, 'What was that girl whistling?' you would probably be able to say what the tune was (if you recognised it.)
So all sorts of information can go into your brain without your necessarily being aware of it. Perhaps that explains the experience of déjà vu (when you have that strange feeling that something has happened exactly like this before). While your brain is recording an experience it might make a mistake and tell you that it's not recording but remembering.
Maybe. No one knows much about this sort of thing. Let me know if you find out any more about it.